The testing problems of concurrent systems include those of sequential programs, but there are two additional difficulties: the scheduling of tasks may alter the behavior, making tests misleading; testing may be conducted at an early stage of development, by users who are not software experts. Concurrent process systems can be modeled by a collection of finite-state transducers, in a way that displays their unique problems. The specification languages PAISLey and Gist approach the definition of concurrent systems differently, but both permit users to execute partially defined systems. The declarative language PROLOG, although not explicitly designed for concurrent programming, exhibits similar characteristics. Prototype execution has some unexpected implications for testing, and for final implementation.